/Engine/Source/Runtime/Core/Public/Serialization/Archive.h
SerializeCompressedNew can read existing data written by old SerializeCompressed calls.
void SerializeCompressedNew ( void * V, int64 Length, [FName](API\Runtime\Core\UObject\FName) CompressionFormatToEncode, [FName](API\Runtime\Core\UObject\FName) CompressionFormatToDecodeOldV1Files, ECompressionFlags Flags, bool bTreatBufferAsFileReader, int64 * OutPartialReadLength )